Study On Heat Transfer And Temperature Uniformity Of Vapor Chamber In A Compound Liquid Cooling System

To guarantee the high-efficiency performance and improve the service life of the power batteries,the battery thermal management system(BTMS)is utilized for improving the heat dissipation efficiency and temperature uniformity.In the liquid cooling system,the temperature rise of the coolant would result in a large temperature gradient between the inlet and outlet,which would decrease the temperature uniformity of the battery modules.In this paper,a compact and efficient compound liquid cooling system consisting of the vapor chamber and the cold plate was designed which was aimed to improve the temperature uniformity of the heat source surface and simplify the flow channel design of the cold plate.However,the effect mechanism of the uneven temperature distribution at the cooling surface on the heat and mass transfer inside the vapor chamber during the phase change process was not clear.And few studies have focused on the influence of the temperature rise of the coolant on the temperature uniformity of the vapor chamber in a practical compound liquid cooling system.In this thesis,the thermal performance and temperature uniformity of the vapor chamber were experimentally studied in the compound liquid cooling system under different operating conditions.The "suppression ratio" was introduced as an evaluating indicator to assess the temperature uniformity of the heat source surface.Then,a numerical heat transfer model coupling with the convection,heat conduction and phase change in the compound liquid cooling system was established.The numerical simulation results revealed that the heat and mass transfer inside the vapor chamber would affect the temperature uniformity of the heat source surface in this compound liquid cooling system.On this basis,the thermal performance and temperature uniformity of the vapor chamber under different structural and operating parameters were computed,and the influence of above parameters on the temperature uniformity of the vapor chamber analyzed.Finally,based on the theory of the vapor flow and heat transfer inside the vapor chamber,the empirical correlation of the suppression ratio with the structural and operating parameters was obtained by the experimental and simulation results,which provided a scientific basis for the design and operation of the compound liquid cooling system.In summary,the research results of this paper are as follows:(1)Experimental study on the thermal performance of the vapor chamber in compound liquid cooling system.A compound liquid cooling system combining a grooved aluminum vapor chamber with a one-through flow cold plate was proposed and studied.The heating power,coolant flowing rate,filling ratio and tilting angle of the vapor chamber were investigated,which had an impact on the thermal resistance and temperature difference on the heating surface.The results showed good thermal uniformity of the vapor chamber with small thermal resistance at small filling ratio.As the heating power increased and the flowing rate of the coolant decreased,the suppression ratio of the heating surface would increase correspondingly,which indicated that the vapor chamber can effectively suppress the impact of the temperature rise of coolant on the temperature uniformity of the heating surface.In addition,the experimental results showed that the vapor chamber at the positive tilting configuration had a better thermal performance than that at the negative tilting configuration.(2)Numerical study on the heat and mass transfer of the phases inside the vapor chamber in a compound liquid cooling system.A three-dimensional model coupled with the heat conduction,heat convection and phase change process was established and computed.Based on the volume of fluid(VOF)model and evaporation-condensation model,the pressure,temperature,volume fraction and flow characteristics of the phases were investigated during the start-up process from 0 s to 60 s.When the pressure and temperature filed of phases tended to be stabilized,the computed results agreed well with the experimental results,which indicated the reliability of the numerical model.During the start-up process,the temperature rise of the coolant between the inlet and outlet of the cold plate would be enlarged gradually,which would cause the temperature of the vapor chamber on the right side higher than that on the left side.As a result,the condensating region was mainly distributed in the left side of the chamber,and the vaporizating region was largely distributed in the right side of the chamber.There was a certain pressure difference between the the left and right side inside the vapor chamber,which would drive the vapor flowing from the high temperature region to the low temperature region.The numerical results revealed that the heat and mass transport of phases along the longitudinal channel was the main reason for the improvement of the temperature uniformity of the heating surface for the vapor chamber.(3)The influence of the operation parameters on the thermal uniformity of the vapor chamber.Based on the three-dimensional coupled heat and mass transfer of the compound liquid cooling system,an improved transient calculation method was adopted and verified to reduce computation cost.The heat and mass transport process of phases inside the vapor chamber was computed,which was aimed to obtain the thermal uniformity of the heating surface under the effects of the heating power,tilting configuration and flowing rate.The results show that the temperature difference of the heated surface caused by the temperature rise of the coolant can be effectively suppressed under different heating powers and flowing rates.In the positive tilting configuration,the buoyancy force promoted the directional flow of vapor phase inside the vapor chamber along the longitudinal channel,which improved the temperature uniformity of the heating surface.In the negative tilting configuration,the phase change process of the working medium deteriorated,and the two-phase flow was obstructed inside the chamber,which resulted in the decrease of the temperature uniformity of the heating surface.(4)The influence of structural parameters on the thermal uniformity of the vapor chamber in the compound liquid cooling system.Based on the three-dimensional model of the coupled heat and mass transfer,the influence of channel parameters(length,width,height)on the thermal performance of the vapor chamber were investigated.The results showed that increasing the height and width of the channel could increase the flow area of the phases in the longitudinal direction,which could improve the thermal uniformity of the heating surface,but also lead to the increase of the thermal resistance.Moreover the increase of channel length would result in the decrease of the phase transport ability along the grooved channel,which would lead to the decrease of thermal uniformity of the heating surface.Combining with the suppression ratio of the heaing surface under different operating parameters and structural parameters,the empirical correlation of the suppression ratio was obtained,which was used to predict the comprehensive influence of structural parameters(channel length,width,height)and operating parameters(filling ratio,flowing rate of the coolant,tilting angle)on the thermal uniformity of the heating surface of the vapor chamber in the compound liquid cooling system.